Purification of lower olefins



Patented Feb. 21, 1950 PURIFICATION OI LOWER OLEI'INS Alfred W. Francis, Woodbury, N. 1., assigncr to Socony-Vacuum Oil Company, Inoo corporation of New York rporated. a

No Drawing. Application October 8, 1947, Serial No. 778,733

9 Claims. (Cl- 260-877) This invention relates to a process for purifyingpropylene and l-butene by selective formation of a liquid complex with silver nitrate. It has been round that, within certain limits of temperature and olefin concentration, these two oleflns will, on contact with solid silver nitrate, form liquid complexes which are insoluble in liquid hydrocarbons.

The phenomenon is unique in that it appears to be limited to the two olefins specified and that it is extremely selective. The complex forms only with the olefins specified and these are completely released on reduction in pressure below the critical values noted below. The process differs in these respects from the previously known methods of scrubbing gaseous mixtures with concentrated aqueous solutions of silver nitrate or certain other metal salts. In these earlier procasses. the capacity of the aqueous solutions is much less than that of the present absorbent, and they required heating or evacuation or extraction with a non-aqueous solvent for substantial recovery of the absorbed olefins. Even when such expedients are employed, complete release of olefins is not achieved. Release of pressure from the complexes of this invention results in complete release of oleflns contained in the complex, regenerating the solid silver nitrate.

It is accordingly the principal object of this invention to purify mixtures of propylene or l -butene in admixture with other normally gaseous compounds. The mixture is subjected in either gaseous or liquid state to' contact "with solid silver nitrate under pressure and tempera-.- ture conditions at which the liquid complex oi. silver nitrate with the olefin is stable. Preferably the mixture is contacted in the liquid state since the pressures and temperatures required are near those at which the olefins liquefy. The liquid complex is immiscible with hydrocarbons and is thus readily separated from the contaminants normally associated with the oleflns.

The impurities in the propylene or l-butene may be hydrocarbons such as propane, ethane, butanes, ethylene, isobutene, etc.. or any. gas such as air or carbon dioxide, or liquid inert to silver nitrate. This excludes hydrogen sulfide, mercaptans, hydrogen halides, and, ammonia.

Preferably acetylene and butadiene should be.

tion,aswillbeunderstoodbythoseskilledinthe 66 2 art. The data reported herein were obtained by a batch type operation.

The apparatus consists preferably of a vertical stirring autoclave made of stainless steel or other material nonreactive with silver nitrate. The autoclave is charged with crystals of silver nitrate and then the above mentioned charge stock as liquid, and stirred until all of the crystals have dissolved. The agitation is stopped, and the upper liquid layer of undissolved hydrocarbon is then discharged through a pipe extending down from the top to a point just above the interface. A means for adjusting this position, including a sight glass must be provided. The hydrocarbon discharge still vcontains oleflns, which may be concentrated by other means for recycling.

The dissolved propylene or l-butene-is then exhausted, using stirrin preferably by another outlet from the top of the autoclave, its entrance being protected from spattering of the liquid complex, as it loses its gas. The first portion of this gas is used to purge the impurities from the vapor phase and the small portion of upper layer not removable as liquid. The remaining gas is substantially pure propylene or l-butene. The residue consists oi finely divided crystals of silver nitrate, which is available for extraction of more propylene.

Example I one-half atmospheres with stirring mitu the disa appearance of solid silver nitrate indicated completion of complex formation. The presence of two separate liquid phases was clearly discernible through a sight glass in the autoclave. The liquids were then discharged from the autoclave as described above. Except for the small amount .used in purging, all the propylene was obtained in pure state, free of propane.

In view of the belief that the present complexes are unique and since no published description of substances of similar nature is known to me, a detailed description of the complexes of this invention is given.

The complexes are easily flowing liquids of density 1.7 to 3.0 grams per milliliter depending on composition. They behave like inorganic substances, being completely miscible with water, and substantially immiscible with orsanic liquids amount of the latter dissolved varies with the temperature, increasing on cooling. The freezing point is near C., but it may be readily supercooled as low as -32 C. On raising the temperature, the complex loses propylene, depositing crystals of silver nitrate. The liquid phase disappears completely at 38 C. (and probably slightly lower) even in presence of liquid propylene. Near that temperature, however, the equilibrium is slow so that the liquid may exist temporarily as high as 60 C.

The dissociation pressure of the complex is given approximately by the equation,

' g P=5.331283/T (where P is pressure in atmospheres and T is absolute temperature) in comparison with the vapor pressure of propylene, which is log P=4.357-983/T (J. Am. Chem. Soc., 55 4342 (1933)). The two curves intersect at about 36 0., so' that the complex has no stable existence above thattemperature. The equation was calculated from the relative vapor pressures at 0 C. (0.75) and at 36 -C. (1.0). The complex shows the phenomenon of seeming to freeze on warming and to melt on cooling.

At 0 C. the dissociation pressure of the complex is about three-fourths of the vapor pressure of propylene, so that it isin equilibrium with about 0.75 mol fraction. An impurity of over 25 per cent propane prevents the formation of the complex at this temperature. It can be considered as extracting the propylene from the complex. Even another olefin, isobutene, has the same effect; and apparently it is insoluble in the propylene complex. Ethylene likewise fails to form a liquid complex, and apparently no solid complex over temperatures from 25 C. down to that of liquid air, although it has not been demonstrated that ethylene is insoluble in the propylene complex. Both olefins are readily soluble in aqueous solutions of silver nitrate.

The l-butene complex has a behavior similar to that of the propylene complex except that the maximum temperature of permanent stability is about 25 C. instead of 36 C., and it does not freeze at 'I8 C., although it becomes viscous at that low temperature. The maximum mol ratio of l-butene, 1.54, is about the same as with propylene. The complex is considerably more tolerant of isobutene, so that a mixture of 40 per cent l-butene and 60 per cent isobutene will form a liquid complex with the former olefin at temperatures below 0 C.

Since the complex is not formed with concentrations of propylene below about 75 per cent (or possibly 70 per cent at low temperatures), it is not applicable for separating propylene from low concentrations, but only for last stage purification.

0n the other hand, the use of the complex has two important advantages over the use of aqueour solutions for purifying propylene and l-bu- Example I! A mixture of 40 mol per cent l-butene and 60 mol per cent isobutane was charged to the autoc ve with silver nitrate crystals in a weight ra io of 1.25 grams of hydrocarbons per gram of the silver salt. The contents of the autoclave were stirred at 0 C. and 1.5 atmospheres for a period of time sufilcient to cause complete disappearance of solid silver nitrate. The contents were discharged as described above. Except for the amount required for purging, the olefin was recovered in the free state.

I claim:

1. A process for separating a primary normal olefin having three to four carbon atoms per molecule from a fiuid mixture, which mixture contains at least about mol per cent of said olefin when said olefin is propylene and contains at least about 40 mol per cent oi said olefin when said olefin is l-butene, which comprises: contacting said mixturewith solid silver nitrate to form a liquid complex of said olefin with silver nitrate, said contacting being efiected under conditions of temperature and pressure at which said liquid complex is stable; separating said complex from the remainder of the mixture while maintaining said temperature and pressure conditions; and dissociating said complex to silver nitrate and said olefin, thereby separating said olefin.

2. A process for separating propylene from a fluid mixture containing at least about 75 mol per cent of propylene, which comprises: contacte said mixture with solid silver nitrate at a temperature not substantially above 36 C. and a pressure in excess of the dissociation pressure of the resultant liquid complex of propylene and silver nitrate; separating said complex from the remainder of the mixture while maintaining said complex in stable form; and dissociating said complex to silver nitrate and propylene, thereby separating propylene.

3. A process for separating l-butene from a fiuid mixture containing at least about 40 mol per cent of 1-butene,' which comprises: contacting said mixture with solid silver nitrate at a temperature not substantially above 25 C. and a pressure in excess of the dissociation pressure of the resultant liquid complex of l-butene and silver nitrate; separating said complex from the remainder of the mixture while maintaining said complex in stable form; and dissociating said complex to silver nitrate and l-buteue, thereby separating l-butene. p

4. The process of claim 1 wherein the mixture is a liquid mixture.

5. The process .of claim 2, wherein the mixture is a liquid mixture.

6. The process of claim 3, wherein the mixture is a liquid mixture.

7. The process or claim 1, wherein said fiui mixture contains said primary normal olefin and materials inert to silver nitrate.

8. A process for separating propylene irom a mixture containing about 25 mol per cent of propane and about 75 mol per cent of propylene, which comprises: contacting said mixture withll solid silver nitrate at about 0 C. and about 5.5

5 atmospheres; separating the resultant liquid complex of propylene and silver nitrate from the remainder oi. said mixture while maintaining said complex in stable form; and dissociating said complex to silver nitrate and propylene, thereby 5 separating propylene.

9. A process for separating l-butene from a mixture containing about 60 mol per cent of isolbutane and about 40 mol per cent of l-butene, which comprises: contacting said mixture with 10 solid silver nitrate at about 0 C. and about 1.5 atmospheres; separating the resultant liquid complex of l-butene and silver nitrate from the remainder of said mixture while maintaining said complex in stable form; and dissociating said 1 complex to silver nitrate and l-butene, thereby separating l-butene.

ALFRED W.

6 nnmnrmcas orrnn The following references are of record in the file of this patent: x

UNITED STATES PATENTS Number Name Date 2,077,041 Davis et a1 Apr. 13, 1937 2,289,773 Gilliland July 14, 1942 2,395,956 Soday Mar. 5, 1946 OTHER REFERENCES Gilliland et al., Jour. Amer. Chem. 800., vol. 61,

pages 1980-62 (1939). 

1. A PROCESS FOR SEPARATING A PRIMARY NORMAL OLEFIN HAVING THREE TO FOUR CARBON ATOMS PER MOLECULE FROM A FLUID MIXTURE, WHICH MIXTURE CONTAINS AT LEAST ABOUT 75 MOL PER CENT OF SAID OLEFIN WHEN SAID OLEFIN IS PROPYLENE AND CONTAINS AT LEAST ABOUT 40 MOL PER CENT OF SAID OLEFIN WHEN SAID OLEFIN IS 1-BUTENE, WHICH COMPRISES: CONTACTING SAID MIXTURE WITH SOLID SILVER NITRATE TO FORM A LIQUID COMPLEX OF SAID OLEFIN WITH SILVER NITRATE, SAID CONTACTING BEING EFFECTED UNDER CONDITIONS OF TEMPERATURE AND PRESSURE AT WHICH SAID LIQUID COMPLEX IS STABLE; SEPARATING SAID COMPLEX FROM THE REMAINDER OF THE MIXTURE WHILE MAINTAINING SAID TEMPERATURE AND PRESSURE CONDITIONS; AND DISSOCIATING SAID COMPLEX TO SILVER NITRATE AND SAID OLEFIN, THEREBY SEPARATING SAID OLEFIN. 